This invention relates to franking machines in which postage values used in franking mail items are metered and an account is maintained of the value of postage issued by the franking machine.
In known franking machines utilizing electronic circuits to carry out accounting and control functions in relation to use of the machine in franking mail items, non-volatile memory devices are provided to store accounting data. Such accounting data usually comprises a value of credit entered into the machine and available for use in issuing postage values for franking mail items, an accumulated value of postage used in operation of the machine, an items count comprising the number of items for which a postage value has been issued and a high items count comprising the number of items for which a postage value in excess of a predetermined value has been issued. The non-volatile memory may also store parameters used in operation of the machine and it has been proposed to store a history of faults occurring in the machine. As is well known in the franking machine art, it is essential that the accounting data is stored in a non-volatile manner because reliance is placed upon this accounting data by the postal authority for charging the user of the machine for postage value issued.
Memory devices commonly used in electronic franking machines are dynamic semi-conductor devices which retain data therein only so long as power is applied to the devices. When power to these devices is terminated any data residing in the storage location of the device is lost. Termination of power to a franking machine may occur due to a normal power down of the machine or due to an unpredicted interruption of a mains electricity supply to the machine. Accordingly in order to ensure that data is retained in the memory devices when power to the machine is terminated, a battery is provided for each memory device to maintain power at all times to the device and thereby prevent loss of data. Back up of the data is provided by storing replications of the data in two separate memory devices, each with its own dedicated battery back up power supply. Further in order to ensure that integrity of the accounting data can be maintained in the event of a fault condition relating to the memory devices or to the writing and reading of data into and from the memory devices, the data is replicated in each memory device. Thus usually, for each item of accounting data, each of the two memory devices has two registers so that four copies of each item of data are stored. The need to provide a battery to back up the power supply to each memory device is inconvenient and adds cost to the machine. To ensure that data is retained in the memory devices for a time sufficient to meet the specified requirements of postal authorities, worst case conditions must be used in calculating battery capacity needed to meet the specified requirements. The result is that the batteries and associated circuitry take up substantial areas of the printed circuit boards of the machine. It becomes necessary to compromise in the choice of memory device to be used and the worst case current drain of such devices becomes a critical factor in choice of device. A consequence is that memory devices with a smaller data storage capacity than desired have to be used in order to meet the conflicting requirement of battery capacity.
In an attempt to overcome the need for battery back up of power supply to the memory devices it has been proposed to use devices known as electrically erasable programmable read only memories, E.sup.2 PROMs. Such devices have been intended for use in a read only mode to store invariable data such as program routines utilized for operation of equipment. The data is written into the memory device initially and remains unchanged thereafter. While it is possible to write data into these semi-conductor devices, the devices are able to operate only for a limited number of erase/write cycles. Accordingly they can only be used in situations where re-writing of data is required infrequently and have not been suitable for use for the purpose of storing and retaining data which is frequently re-written during operation of equipment such as accounting data in franking machines.
While E.sup.2 PROM devices would be convenient to use for storage of account data in a franking meter due to their ability to store data for up to ten years without energization by any power, accounting in a franking meter is carried out for every franking cycle and involves writing of new accounting data to the memory device during each franking cycle. The operational life of a franking meter is generally specified as requiring the meter to be capable of carrying out 4.times.10.sup.6 franking cycles, however E.sup.2 PROM devices which are generally available at the present time have an operational limit of 10-100.times.10.sup.3 read/write cycles. Even one particular device of higher performance which is expensive has a limit of 2.times.10.sup.6 write cycles. A further disadvantage of E.sup.2 PROM memory devices currently available is that the writing cycle for writing data in the memory is long compared with dynamic memory devices and this limits the use of such devices to situations where only small amounts of data are required to written in each write cycle. In some franking machines, the length of write time of the E.sup.2 PROM devices may preclude use of such devices.